CC composite is advanced materials having an excellent specific strength, specific stiffness and abrasion resistance at an ultra high temperature.
CC composite is generally prepared by impregnating pitch into carbon fibers. Since CC composite is used under severe conditions involving high temperature, pressure and speed, the graphite crystal structure of the matrix formed by an impregnated pitch should be brought to have a fine mosaic or isotropic structure, not a lamellar structure, and densification thereof is also needed. Further, the interfacial adhesion between the matrix formed by heat treatment of the impregnated pitch and carbon fiber needs to be improved. When the graphite crystalline structure is a lamellar structure, the CC composite can be seriously damaged, as the ‘a’ and ‘b’ planes of the graphite crystal are likely to be peeled off by fluids having high temperature, flow rate and pressure. When the interfacial adhesion between the carbon fiber and the matrix is weak, fluids having high temperature, flow rate and pressure is likely to be concentrated on such weak interfacial part, then facilitates oxidation thereof, and accelerates peeling off occurred in ‘a’ and ‘b’ planes due to stress concentration, resulting in significant abrasion of the CC composite.
In manufacturing CC composite, an impregnating pitch is used in a molten form at a temperature of 300-400° C. where it shows the optimum viscosity for high densification of a CC composite preform. An impregnating pitch having the lower viscosity shows the lower carbonization yield, because it contains relatively more volatile components having a low molecular weight and a low boiling point. Although the impregnating pitch having a low viscosity effectively penetrates into a CC composite preform during the impregnation process, thereby having improved impregnation efficiency, carbonization yield thereof by a carbonization process at 1,000° C. or more in an inert atmosphere followed by a graphitization process at 2,000° C. or more in an inert atmosphere becomes low, and the desired density in the final CC composite can be hardly achieved. Therefore, in order to improve the density of the CC composite, a cycle of an impregnation, carbonization and graphitization process should be repeated more than 5 times.
An impregnating pitch for CC composite has been prepared by heating a pitch in an inert atmosphere for improving the degree of graphite crystallinity and an impregnating efficiency, thereby a spherical anisotropic mesophase which is easily transferred to graphite crystals can be formed, and then by removing volatile compounds having a low molecular weight and a low boiling point so as to increase the carbonization yield.
Methods of manufacturing an impregnating pitch for CC composite have been disclosed in this field of art. For example, Korean patent No. 0653929, Japanese patent laid-open Nos. 1998-145392 and 1994-248274 describe methods for preparing an impregnating pitch which comprises up to 50% of lamellar-structured mesophase that is easily transferred to graphite crystals, by carrying out a carbonization process at 1,000° C. or more and a graphitization process at 2,000° C. or more for improving carbonization yield, and then heat treatment for developing graphite crystals. Although the mesophase-containing pitch obtained from the conventional methods has graphite crystals well-developed through the final graphitization process at 2,000° C. or more, the matrix portion of the CC composite becomes to have a lamellar structure owing to the well-developed graphite crystals, resulting in rapid abrasion of the CC composite by fluids having high temperature, speed and pressure. Further, by polycondensation of aromatic compounds present in pitch occurred during the heat treatment of the pitch, the heat-treated impregnating pitch becomes a higher molecular weight polymer with an increased viscosity of at least 100 cP, so that the penetration of the impregnating pitch into the CC composite becomes difficult and the impregnation efficiency becomes lowered, resulting in hindering the densification of the CC composite.
Further examples of methods of manufacturing CC composite, other than those using an impregnating pitch, include: a method using a thermoplastic resin, a thermosetting resin and an isotropic pitch (Japanese patent laid-open No. 1999-130553A2); a method using a thermosetting resin and a mixture of a common pitch powder added with a graphite powder (Korean patent publication No. 1995-0011212); and a method wherein a pyrolized carbon is deposited onto a carbon fiber preform (Korean patent laid-open No. 2000-0049126, Japanese patent laid-open No. 2000-086382A2). However, these methods also have drawbacks such as difficulties in the densification of CC composite, and particularly a method using deposition of pyrolized carbons onto a carbon fiber requires a long manufacturing period and high production cost, thereby being economically disadvantageous.
Properties of an impregnating pitch demanded for CC composite manufacture include a low viscosity and a high carbonization yield, in view of the impregnation efficiency and densification efficiency of the final CC composite product. Since the low viscosity and the high carbonization yield of pitch are conflicting properties each other, there still remains a need for overcoming such conflicting properties so as to develop a high performance CC composite at a low cost.